The present invention relates to an apparatus for sampling particle-laden gas flows and concentrating trace organic species within that flow for subsequent analysis by gas phase detectors.
Environmental sampling for target chemical species often requires knowledge of gas/particle phase partitioning. The partitioning of target chemicals such as pollutants between the gas phase atmosphere and the surfaces of particles has implications in study and analysis of transport model predictions, atmospheric reaction modeling, and evaluation of pollutant formation/evolution mechanisms. The actual partitioning of these target chemical species between gas phase and condensation/absorbed solid phase as a function of their vapor pressure, concentration and solubility are important aspects in such studies. System temperature and characteristics of the absorptive particle surfaces are also important factors in partitioning. Sampling methods to determine the concentration and/or partitioning of the target chemicals can affect the actual partitioning. Sampling systems which operate with a particle filter allow accumulation of particulate solids during the sampling episode, which may last over 3 hours. These particles can act as sorptive surfaces for gas phase targets through condensation or absorption/reaction phenomenon (“blow-on”), thereby transferring the target analytes to the solid phase. (See, for example, articles by Cotham & Bidleman in Eniron. Sci. Tech. 24, p 342 and Gundel, et al., Atmos. Environ. 29, p. 1795 (1995).) Alternatively, the filter surfaces can lose particle-bound volatile target analytes to the downstream collection media (“blow-off”), thereby biasing results toward gas phase partitions as described by Eatough, et al., (Organic Chemistry of the Atmosphere CRC Press, 1999). These sampling artifacts introduce bias into determinations of phase partitioning of volatile and semi-volatile compounds.
For analysis of target species, concentration methods are often employed to bring the quantity of target above the analytical detection limit of the apparatus. This includes methods such as cryogenic focusing, pressure swing absorption as described by Keefer in U.S. Pat. No. 4,968,329, which is incorporated herein by reference, and sorbent collection as described by Sides, et al., in U.S. Pat. No. 5,014,541 and Pleil, et al., U.S. Pat. No. 5,447,556 (extraction/solvent evaporation), both of which are hereby incorporated by reference. These methods teach gas phase target concentration, but do not address maintaining and preserving the distinction in the trace organic species phase (vapor or condensed on particles) as exists at the point of sampling.
There are some sampling methods that address phase partitioning of targets. The standard EPA (Environmental Protection Agency) method train (U.S. EPA, Test Method 0032A) consists of a filter housing through which the particle-laden gas stream passes followed by an absorbent media. This method can provide a gas versus particle phase distribution, but can suffer considerable bias from the blow-off and blow-on phenomena discussed above, and, hence, is not purported as a method for discerning analyte phase partitioning. As an alternative, denuder sampling has been developed in an effort to continually separate gas phase targets from the solid phase targets. Annular denuders consist of one or more channels through which both gasses and particles pass. The denuder surfaces may be coated with a thin layer of fine absorbent. The denuder principle relies on the relatively faster diffusion of the gas phase target to the denuder wall than that of the exposure of particles to the wall in this laminar flow regime. In this manner, the gas phase components are separated from the particle stream. In most denuder applications, the particle stream is filtered and followed by a sorbent that catches target analyte blow-off. The denuder surface can be solvent-extracted and analyzed for the target components in the vapor phase or vapor phase components can be determined simply by the difference of the post-denuder filter catch and a separate total sample catch. Bias in denuder operations results from fine particle diffusion to denuder walls (estimated at 10% for 0.1-0.05 m particles), volatilization of target analytes from particles during transit through the denuder channels, and breakthrough of the volatiles from the denuder to the downstream adsorbent.
A method of particle segregation of gas streams is taught to create a particle-free stream and a particle-rich stream in a method (U.S. Pat. No. 5,746,789 to Wright and Crouch, which is incorporated herein by reference). That method of segregation increases the relative gas to particle velocity, thus exacerbating concerns over particle blow-off and blow-on.
Many filters have been taught as providing means for avoiding pressure drop caused by the buildup of particles of simply as means of particle filtration. Some moving filters include self-cleaning processes (U.S. Pat. No. 5,560,835). The references show intent only to limit pressure drop due to particle buildup on the filter. None of the references teach a method for limiting gas flow through previously filtered particles, thereby minimizing blow-off and blow-on complications.